European Patent Application No. 0 477 400 discusses a system for an adaptive, mechanical tolerance compensation, acting in the lift direction, for a path transformer of a piezoelectric actuator for a fuel injector. In this case, the actuator acts on a master (transmitter) piston connected to an hydraulic chamber, and a slave (receiving) piston, moving a mass to be driven and positioned, is moved via the pressure increase in the hydraulic chamber. This mass to be driven is, for example, a valve needle of a fuel injector. The hydraulic chamber is filled with an hydraulic fluid. When the actuator is deflected and the hydraulic fluid in the hydraulic chamber compressed, a small portion of the hydraulic fluid leaks at a defined leakage rate. In the rest phase of the actuator, this hydraulic fluid is replenished.
German Published Patent Application No. 195 00 706 discusses a hydraulic path transformer for a piezoelectric actuator in which a master piston and a slave piston are lying on a common axis of symmetry and the hydraulic chamber is located between the two pistons. A spring, which presses apart the master cylinder and the slave piston, is located in the hydraulic chamber, the master piston being prestressed in the direction of the actuator and the slave piston being prestressed in a working direction of a valve needle. When the actuator transmits a lifting movement to the master cylinder, this lifting movement is transmitted to the slave piston by the pressure of a hydraulic fluid in the hydraulic chamber since the hydraulic fluid in the hydraulic chamber is not compressible and during the short duration of a lift only a very small portion of the hydraulic fluid is able to escape through ring gaps between the master piston and a guide bore, and a slave piston and a guide bore.
In the rest phase, when the actuator does not exert any pressure on the master piston, the spring pushes apart the master piston and the slave piston, and, due to the produced vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the ring gaps and refills it. In this manner, the path transformer automatically adapts to longitudinal deformations and pressure-related extensions of a fuel injector.
In other systems, hydraulic fluid may evaporate during a relief period in which no high pressure prevails in the hydraulic chamber. However, gas is compressible and generates an appropriately high pressure only after a substantial reduction in volume. The master cylinder may then be pressed into its guide bore without a force being transmitted to the slave piston.
This danger exists, in particular, in a fuel injector used for injecting gasoline as fuel, in those instances where the gasoline is also used as the hydraulic fluid. This danger is increased even further in the case of a directly injecting fuel injector for gasoline once a hot internal combustion engine has been switched off. A fuel-injection system then loses its pressure, and the gasoline evaporates particularly easily. In a renewed effort to start the internal combustion engine, this may lead to the lifting movement of the actuator no longer being transmitted to a valve needle and the fuel injector no longer functioning.
A cavitation of the fuel may occur if the spring exerts a high clamping force upon the master cylinder and the slave cylinder and the movement of the actuator into its original position occurs very rapidly. The vacuum pressure being generated in the hydraulic chamber may then lead to cavitation and to damage of components resulting therefrom.